ALTERATION OF THE CIRCADIAN-RHYTHM OF INTACT PARATHYROID-HORMONE FOLLOWING A 96-HOUR FAST

OBJECTIVE PTH(1-84) secretion in normal mate subjects follows a circad ian rhythm. The control of this rhythm is multifactorial with both neu roendocrine and chemical influences. The aim of this study was to asse ss the effect of a 96-hour fast on the circadian rhythm of PTH(1-84), serum calcium, phosphate and nephrogenous cAMP (NcAMP), an index of PT H(1-84) bioactivity. DESIGN Blood samples for estimation of all analyt es were obtained over a 24-hour period at 30-minute intervals. Urine s amples were obtained 4 hourly during the daytime and overnight. Each s ubject was studied on two occasions after being randomized to either ( a) normal hospital diet or (b) a 96-hour fast with water freely availa ble. SUBJECTS Six healthy adult males aged between 28 and 40 years, me an 32 years. MEASUREMENTS PTH(1-84) was measured by an inhouse immunor adiometric assay. Serum calcium, phosphate, albumin, creatinine and ur inary creatinine were measured by standard automated techniques. Calci um was adjusted for albumin. Plasma cAMP was estimated by a commercial method and urine cAMP by in-house radioimmunoassay and NcAMP obtained by calculation. Rhythm parameters were analysed by cosinor techniques . RESULTS There were alterations in the circadian rhythms of serum pho sphate, PTH(1-84) and NcAMP following a 96-hour fast. Pasting abolishe d the nocturnal rise in phosphate, PTH(1-84) and NcAMP but had little effect on the pattern of adjusted calcium over a 24-hour period. The m ean concentrations of serum phosphate, adjusted calcium and NcAMP decr eased significantly following the fast and mean PTH(1-84) increased du ring day time. CONCLUSIONS Pasting for 96 hours significantly alters t he circadian rhythm of PTH(1-84) secretion by lowering the mean calciu m concentration and attenuating the circadian rhythm of serum phosphat e.